(submitting because it's easier, and lacks a character limit.)

I think the thing about BBS is that everyone involved makes mistakes! In particular, I can think of some really cruel lines that Aqua and Ven have - ones that are pretty hard to misinterpret, and that they had to have realized would hurt their friends. ("You've let it go to your head," from Ven, isn't that bad at face value, but they've been friends for four years - I'm sure he knows that she Does Not like being called bigheaded; "You'll go astray again," from Aqua, isn't entirely unjustified in the english version but i won't bore you with that rant, but it is something that cuts straight to Terra's worries (the ones he's just told her about).) Aqua has to learn that maybe she isn't giving her friends enough credit (Ven, especially) and that doing what she thinks is best for them isn't always the best option.

Ven's pretty impulsive - I agree that he's not very much to blame, but he does make mistakes too! (Here's one - why did he believe the mysterious stranger in his bedroom?) And Aqua has to clean some of those messes up.

"But the fact of the matter is, she spend her story cleaning up messes that Terra made. In actuality, honestly, if she was ordered to do so, if her mission was bringing Ven home, she’d have been done AGES earlier."

But that... wasn't her orders?

1) She was ordered to keep an eye on Terra. Which she did! She probably shouldn't have believed what Maleficent said so readily, but she did.

2) She was ordered to find Master Xehanort, and destroy the Unversed. Eventually happened.

3) She was ordered to bring Ven home. Note that when she told him to, she was kind of dismissive of what Ven was saying, and that he ran off - I doubt she was about to knock him out and drag him home.

Also, yeah, Terra's pretty stupid with regards to Xehanort's shit! It's even easier to say that from a player's standpoint: we know that Xehanort is a bad name from the beginning, we've seen the bad things darkness can do in the series, and we can see that Terra's pretty much doomed. But he can't! Terra's a teenager, or close to it (going by Xemnas being "about 30" according to an interview). He and Aqua seem reasonably sheltered (they've been to other worlds, or Ven wouldn't have thought anything of being forbidden to, but they both come across as kind of inexperienced IMO), and Xehanort is presented to Terra as a friend of his Master's.

Terra thinks pretty highly of Eraqus! So of course anyone Eraqus (seemingly) trusts is someone worth trusting, in his eyes. Also note that we never see Eraqus telling Terra why darkness is bad - so he could have just assumed it was his Master's personal bias, and that it couldn't be that bad.

Also, here's the thing - you talk about Aqua not deserving her fate - and she doesn't! None of them do, in BBS. But that's not how she sees it. Aqua is a character who would do anything for her friends or to uphold her duties. If anyone else who wasn't an enemy was in the position Terra was in (falling into Darkness, fighting from the inside, with Aqua knowing that being surrounded by darkness would basically be a death sentence), she probably would have done the same thing.

BBS is a game where there would be a lot fewer problems if the characters in it slowed down to listen to one another, and if they explained why the others should do something. I guess you could say that a failure to communicate is what doomed TAV.